[#53] datastore_upload don't check hash for new files

datastore_upload was skipping new resource files because their hash
wasn't changed, so the files would never be datastored. Fix by not
checking the hash for new files. Fixes #53.
Merge pull request #54 from okfn/53-bug-datastore_upload-skips-new-files

[#53] datastore_upload don't check hash for new files